This disclosure relates to composite electrode materials for electric lamps and methods of manufacture thereof.
The standard electron emissive coating currently used in a majority of electrodes of commercial fluorescent lamps contains a mixture of barium, calcium, and strontium oxides (“triple oxide emissive mixture”). Since these oxides are highly sensitive to ambient carbon dioxide and water, they are generally placed on the lamp electrodes initially as a wet mixture suspension of barium, calcium and strontium carbonates containing a binder and a solvent. The wet mixture suspension is then “activated” inside the lamp assembly during the manufacturing process by resistively heating the electrodes until the carbonates decompose, releasing carbon dioxide and some carbon monoxide, and leaving behind a triple oxide emissive mixture on the electrode.
However, the triple oxide emissive mixture suffers from several drawbacks. First, the “activation” requires an undesirably high temperature to convert the carbonates to oxides. The conversion of the carbonates to oxides undesirably releases carbon dioxide and some carbon monoxide. Incomplete activation can also lead to lamp performance issues like high ignition voltage, premature cathode breakdown, and loss in light output due to early wall darkening. Additionally, lamps having electrodes coated with the triple oxide emissive mixture have a rather short operating lifetime. Triple oxide emissive mixtures have therefore been substituted with barium tantalate emissive mixtures having various barium to tantalum ratios. The activation of barium tantalate is simple, as it does not require the decomposition of carbonates. “Activation” in this case is needed only to burn out the binder and remove the water vapor. Moreover, barium tantalate permits a higher loading of the cathode than the triple-oxide emissive mixture. The barium tantalate emissive mixtures are generally “activated” in less time and at a lower temperature than the triple oxide emissive mixture. Furthermore, lamps having electrodes coated with the barium tantalate emissive mixtures have a longer operating lifetime than the lamps with the triple oxide emissive mixture. However, a fluorescent lamp containing the barium tantalate emissive mixture generally has a somewhat inferior efficacy compared to the triple oxide emissive mixture. Adsorbed moisture is believed to be one of the reasons leading to dark band formation during the first one hundred hours of lamp operation. In addition, the moisture sensitivity of the barium tantalate emissive mixture gives rise to many serious manufacturing and processing issues. It is therefore generally desirable to develop a composition for discharge lamps which can function efficiently and which can reduce or even eliminate some of the moisture sensitivity issues presented by the barium tantalate emissive mixture.